Interest in diet modification as a means of preventing breast cancer persists despite inconsistent results of epidemiologic studies. Previous studies, however, have focused primarily on single nutrients; they have not addressed the broader issue of whether overall diet or dietary patterns affect breast cancer risk, largely because of conceptual and methodological difficulties in defining and measuring dietary patterns. The proposed research will examine the association between dietary patterns and breast density among participants in the Minnesota Breast Cancer Family Cohort. Specific aims are to: (1) conduct a detailed characterization of dietary pattern intake in the cohort, and (2) examine associations dietary pattern-breast density associations, additionally exploring the role of specific dietary components such as saturated fat, and assessing whether dietary pattern-breast density associations differ by family history of breast cancer. The analysis will include women enrolled in the 1990 follow-up to a 1944-52 family study of breast cancer at the University of Minnesota. Participants were 1st- or 2nd degree relatives of breast cancer probands identified in the original family study, or spouses of male 1st or 2nd-degree relatives of the probands. Principal components analysis will be used to identify food intake patterns in 3,600 women who completed a 153-item food frequency questionnaire. Dietary pattern reliability and validity will be evaluated. Dietary pattern-breast density associations will be examined in approximately 1,875 women >=40 years of age who also provided a recent mammogram. Analyses will be repeated stratifying on family history of cancer, with family history assessed three ways, based on number of relationship to original breast cancer proband, a familial risk score reflecting family history of any cancer and ages at onset, and family history criteria indicating hereditary transmission of cancer. Additional analyses will compare principal components analysis-based measures with alternative methods for measuring dietary patterns. The proposed study will explore dietary patterns and breast density using detailed methods to insure reliable and valid measurement of dietary pattern intake. Findings will have implications for dietary change as a means of reducing breast cancer risk in women at average and at high risk for the disease.